1. Field of the Invention
This invention relates to musical tone synthesis and in particular is concerned with an improvement for producing tones having a time variant spectral content.
2. Description of the Prior Art
An elusive goal in the design of electronic keyboard operated musical instruments is the ability to realistically imitate the sounds of conventional acoustic type orchestral musical instruments. The best results have been obtalned for electronic musical instruments which imitate wind-blown pipe organs and harpsichords. The principal reason for obtaining good imitative results for these instruments is that they are essentially mechanical tone generators. The tone generation is entirely automatic and the musician only actuates on-off switches. With the notable exception of such instruments the tone character of almost all other acoustic musical instruments is a function of certain skills possessed by the musician.
It has long been recognized that with the possible exception of conventional organ tones, almost all tones produced by musical instruments exhibit tone spectra which are time variant. The recognition of the vital characteristic of time variant spectra has motivated the development of electrical musical tone generating systems such as those known by the generic names of "sliding formant" and "FM-synthesizer." Sliding formant tone generators constitute a class of generators which are also called subtractive synthesis. In subtractive synthesis, the fundamental tone source generates more than the ultimate desired tone spectral components and the undesirable spectral components are attenuated, or filtered out, by means of some variety of frequency filter. The FM-synthesizer is of the additive variety in that FM (frequency modulation) is used to add components to a source signal which frequently consists of a simple single frequency sinusoid time function.
The imitation of acoustic orchestral musical instruments using synthesis techniques such as the sliding format or FM-synthesizer has been one of trial and error. One adjusts a multiplicity of tone controls and ADSR envelope controls until an output tone is produced which is judged to be close or somewhat imitative of a particular musical instrument. Such techniques are in juxtaposition with the intuitive procedure in which one first analyzes the tones produced by a selected musical instrument. Based upon such an analysis an analytical model is postulated so that experimentally obtained parameters can be inserted into the analytical mode to synthesize tones that closely imitate the original tones.
The process of analysis, model generation, and synthesis for musical instrument sounds is obviously not an easily implemented procedure judging from the present lack of success except in a few relatively isolated instances. Part of the reason for the lack of success lies in the apparent inability to adequately generate an analytical model which includes many of the subtleties in tone structure imparted by the musician in an effective performance on his instrument. A musician commonly uses a playing technique such that the tonal structure for a given musical note varies with the loudness of the tone. Repeated notes are played with different loudness and tonal structure and these somewhat subtle differences remove the mechanical-like repeated tones produced by most electronic musical tone generators. In general, as the tone level becomes louder, the tone spectra increases in the number and strength of the higher harmonics.
The simplest and most direct technique of imitating the sounds of acoustic musical instruments is to record the sounds on some suitable storage media and then play back the sounds upon demand initiated by
keyboard switches. Such recording techniques have been implemented with various degrees of success for many years. Recently the availability of low cost microelectronic memories and the state-of-the-art digital signal processing techniques has led to the development of musical tone generators which have been given the somewhat misleading generic name of PCM (pulse code modulation). In these systems, the musical sound is digitized and stored in microelectronic memory circuits. Even with the comparatively low cost of such memory devices, the large amount of data storage required for a keyboard musical instrument having the capability of several different tones is still fairly excessive for most musical tone generation systems. A system of the PCM generic type is described in U.S. Pat. No. 4,383,462 entitled "Electronic Musical Instrument."